1. Field of the Invention
The invention relates to improvements in apparatus for near infrared (NIR) analysis for measuring chemical constituents in a quantitative fashion. More particularly, the improvements of this invention allow accurate measurements with inexpensive instruments.
2. Prior Art
There are known in the prior art instruments which measure chemical constituents in a product by means of either reflecting near infrared radiant energy off the product or transmitting infrared energy through the product. These instruments use the phenomenon that certain organic substances absorb energy in the near infrared portion of the spectrum. By measuring the amount of energy absorbed by the product at certain specific wavelengths, precise quantitative measurements of the constituents in a product become available. For example, the measurement of protein content in wheat can be performed in this manner. For general introduction to near infrared quantitative analysis, see the paper presented by Robert D. Rosenthal to the 1977 Annual Meeting of American Association of Cereal Chemists entitled "An Introduction to Near Infrared Quantitative Analysis". Such analysis can be performed on samples within only a few seconds without special sample preparation except for grinding the sample.
Current commercial near infrared instruments use the same generic technology for developing the near infrared energy. The commercial instruments use a tungsten light source that provides radiant energy through the visible, the near infrared as well as the longer infrared portions of the spectrum. This broad spectrum of energy is modified by the use of narrow band optical filters (or gratings or prisms) to allow only a small selected portion of the total spectrum to fall on the object being measured. Thus, the current instruments are not efficient in the use of illuminating energy because only such a very small portion of the total energy provided by the tungsten lamp is used in the measurement. The great majority of the radiated energy from the tungsten lamp is deliberately not used and is dissipated to heat. This causes temperature problems and other penalties in instrument design.
The instrument laboratory of the U.S. Department of Agriculture in Beltsville, Maryland several years ago, performed studies on the use of another type of "light source". In those studies infrared emitting diodes (IRED's are solid state devices that yield energy at very narrow regions of the spectrum compared to a conventional tungsten bulb) were used. The U.S.D.A. work was summarized in the technical paper entitled "Fat Measurement of Ground Beef with a Gallium Arsenide Infrared Emitter" (ASAE publication 1-76 "Quality Detection in Foods"). In the U.S.D.A.'s technical paper there is mentioned the possibility of narrowing the region of infrared energy by putting narrow bandpass filters in front of the IRED. This, however, was directed to an effort for measurement of wavelengths between 920 and 970 nanometers (nm).
In 1978 U.S.D.A. research discovered that precise measurement of the constituents in grain (e.g., protein) can be made at wavelengths slightly longer than discussed in the 1976 paper. The desired wavelengths for protein measurement in grain are between 1000 and 1060 nm.
Previously protein measurement in grain required measurement at wavelengths above 1200 nm. These longer wavelengths forced the use of poor performing PbS light detectors. Thus, the U.S.D.A. discovery that precise measurement of protein in grain can be wavelengths shorter than 1200 nm appears to be of great value because it allows the use of lower cost, and more stable, silicon detectors.
The U.S.D.A. began to search for IREDs with center wavelengths between 1000 and 1060 nm. Unfortunately, only very expensive indium gallium agenide IRED's are available in this wavelength. Conventional IREDs of gallium arsenide provide peak energy well below 1000 nm; normally about 940 nm. Therefore, although the conventional IREDs are of low cost (about a third of 1% of the indium gallium arsenide IRED's) they are not considered useful for protein measurement because the half power bandwidth was limited to between 910 and 970 nm. Half power bandwidth refers to the wavelength that has one-half of the output power as the center wavelength. Normally, IREDs are never used outside the half power bandwidth points.